1. Field of the Invention
The present invention relates generally to an apparatus and method for transmitting a data rate control (DRC) channel in a mobile communication system employing a high data rate (HDR) technique, and in particular, to an apparatus and method for gating or repeating transmission of a DRC channel.
2. Description of the Related Art
In an IS-2000 communication system, in a good channel state, a base station and a mobile station perform power control in order to perform communication at a prescribed data rate. On the other hand, in an HI)R mobile communication system, access terminals (corresponding to the mobile stations in the IS-2000 system) transmit DRC to an access network (corresponding to the base station in the IS-2000 system) at intervals of a predetermined number of slots, and the access network then analyzes the DRCs received from the access terminals and selectively transmits data only to the access terminals in a good channel state after controlling a data rate. The HDR system has a forward link with highly increased throughput, so that it transmits a large amount of data per unit time in a good channel state and transmits a small amount of data per unit time in a bad channel state, by varying a length of a packet using a single common data channel within the limit of the maximum power of the access network. That is, the HDR system transmits data to only one of the access terminals within a concerned access network at a certain time, through a common data channel. The HDR mobile communication system transmits channel state information and data rate control (DRC) information using a DRC channel. Regarding the DRC, the access terminal measures a carrier-to-interference ratio (C/I) of a pilot signal transmitted over a forward link creates the DRC based on the measured C/I, and then reports the created DRC to the access network over the DRC channel.
The pilot signal is used for initial sync acquisition of data transmitted from the access terminal to the access network, for channel recovery, and for indicating reverse power control information. Meanwhile, a reverse data rate indicator (RRI), used in the HDR system, is a signal for indicating a data rate of a reverse link and synchronizing (or time-aligning) frames each comprised of 16 slots. The DRC and the pilot signal are transmitted on a time division multiplexing (TDM) basis. Further, the RRI signal provides an index inserted in a punctured part of an encoded packet of the pilot signal, so as to help the access network to determine a data rate. Table 1 below shows reverse data rate indexes according to reverse data rates.
TABLE 1Data Rate (Kbps)4.89.619.238.476.8153.6Reverse Data Rate123456Indexes
In Table 1, when the reverse link is transmitted at a data rate of 153.6 Kbsp, a 3-bit symbol is transmitted to the access network over a data rate index channel through Walsh symbol repetition using an orthogonal code of length 4. Table 2 below shows an encoding table of the DRC channel.
TABLE 2Required Data Rate (Kbps)4-bit DRCCodeword (8, 4, 4) 38.4000000000000 76.8000111111111 102.4001001010101153.6 (short)001110101010 204.8010000110011307.2 (short)010111001100 614.4011001100110 921.60111100110011228.81000000011111843.21001111100002457.6101001011010Reserved101110100101153.6 (long)110000111100307.2 (long)110111000011Reserved111001101001Null rate111110010110
The access terminal measures a C/I of a signal transmitted from the access network, converts the measured C/I into a codeword associated with the data rate required by the access network in accordance with Table 2, and then reports the results to the access network. As shown in Table 2, the DRC signal is comprised of a 4-bit symbol. The 4-bit symbol is converted into an 8-bit codeword by block coding. The codewords are mapped with the required data rates of the forward traffic channel on a one-to-one basis.
FIG. 1 illustrates a structure of a reverse link transmitter in a common HDR mobile communication system. Referring to FIG. 1, a multiplier 102 channel-spreads a pilot channel 101 by multiplying it by an orthogonal function W04 of length 4 at every slot, and outputs a 1024-chip non-modulated signal having a value ‘0’. RRI 103 is provided to an 8-ary orthogonal modulator 105. The 8-ary orthogonal modulator 105 performs 8-ary orthogonal modulation on the provided RRI and outputs a Walsh symbol. A Walsh symbol repeater 107 repeats the Walsh symbol output from the 8-ary orthogonal modulator 105, and provides its output to a multiplier 109. The multiplier 109 multiplies the Walsh symbol output from the Walsh symbol repeater 107 by an orthogonal function W04 of length 4 at every slot, and outputs 64 chips per slot. A (8,4,4) block encoder 117 block-encodes an input DRC 115. A codeword repeater 119 repeats the block-encoded DRC a predetermined number of times. A multiplier 121 spreads the symbols output from the codeword repeater 119 by multiplying them by an orthogonal function W02 of length 2. A Walsh cover generator 113 outputs an orthogonal function of length 8 corresponding to an input DRC Walsh cover index 111. A multiplier 123 multiplies an output of the multiplier 121 by an output of the Walsh cover generator 113. A multiplier 125 multiplies data output from the multiplier 123 by an orthogonal function W04 of length 4. A time division multiplexer (TDM) 127 time-multiplexes the pilot channel signal, the RRI channel signal and the DRC channel signal output respectively from the multipliers 102, 109 and 125, and provides its output to a complex spreader 141 as an in-phase component. An encoder 131 encodes an input traffic channel signal 129. A modulator 133 performs BPSK (Binary Phase Shift Keying) modulation on the encoded traffic data. An interleaver 135 interleaves the BPSK-modulated data. A data channel gain controller 137 gain-controls the output of the interleaver 135. A multiplier 139 channel-spreads the signal output from the data channel gain controller 137 by multiplying it by an orthogonal function W24 of length 4, and provides its output to the complex spreader 141 as a quadrature phase component. The complex spreader 141 complex-spreads the in-phase component signal and the quadrature phase component signal. A baseband filter 143 baseband-filters the complex-spread signal from the complex spreader 141.
As described above, the pilot channel signal, the RRI channel signal and the DRC channel signal are transmitted to the access network after time multiplexing.
FIG. 2 illustrates a method for transmitting a DRC channel in a general HDR system. As illustrated, each frame is comprised of 16 slots each having a length of 2048 chips (=1.66 msec). In each slot, the pilot channel signal and the DRC channel signal are time-multiplexed in a unit of 46 chips, before transmission. Every user (regardless of to which user group it belongs) continuously transmits the time-multiplexed signal of the pilot channel signal and the DRC channel signal. In this case, there occurs interference between the users.
That is, as stated above, the HDR system continuously transmits the pilot and the DRC to the access network while the data service is connected. Meanwhile, for the high-speed data transmission, information on the C/I and the DRC signal, transmitted over the reverse link, must be correct. However, as shown in FIG. 2, since each user continuously reports the time-multiplexed signal of the pilot signal and the DRC to the access network, there occurs interference between the pilots. If the access network fails to correctly detect the DRC, the access network cannot correctly schedule the data rate and the sector required by the access terminal, so that it is not possible to service additional new users. That is, in the conventional HDR system, when the number of users is increased, it is difficult for the access network to detect the DRC correctly, making it impossible to service new users.
Although FIG. 2 shows a case where the pilot channel signal and the DRC channel signal are subjected to time multiplexing, the same problem may occur even in the case where the pilot channel signal and the DRC channel signal are subjected to code division multiplexing.